The rule everyone half-remembers
"Stay under 183 days and you're fine." It is the most repeated sentence in expat tax — and one of the most misleading. There is no single, universal 183-day rule. 183 days is a common threshold, but the exact number, the period it is measured over, and what even counts as a "day" are set by each country's own law and its tax treaties.
Same number, different periods
Where the 183 days are measured varies more than people expect:
- Some countries use the calendar year.
- Some use a tax year that isn't the calendar year — the UK's runs 6 April to 5 April.
- Some use a rolling 12-month period.
- The United States uses a weighted Substantial Presence Test across three years: this year's days at full weight, last year's at a third, the year before at a sixth.
Count the same trips against two of those systems and you can get two different answers.
183 is rarely the only test
Even where 183 days matters, it is almost never enough on its own. Countries layer on other tests, any of which can make you resident with fewer days:
- a permanent home available to you,
- your centre of vital interests — where your family and economic ties sit,
- a habitual abode,
- domicile or formal registration.
The UK's Statutory Residence Test, for instance, can make someone resident on far fewer than 183 days once enough "ties" are present. And more than 183 days doesn't automatically settle the question either.
Where the OECD's "183 days" actually lives
People often borrow the 183-day figure from tax treaties. In the OECD Model Convention it appears in Article 15, on employment income — as one of three conditions for your home country (not the host) to keep taxing short-term work. It is a test for taxing a paycheck, not for deciding where you are resident. Useful to know, easy to misapply.
When two countries both claim you
If two countries each treat you as resident, the relevant double-tax treaty breaks the tie, usually in the OECD's order: permanent home, then centre of vital interests, then habitual abode, then nationality, then agreement between the two tax authorities. The day count is only one input among several.
So what should you actually do?
- Treat 183 days as a warning line, not a finish line.
- Know which period your country measures over, and when it starts.
- Keep an accurate, day-by-day record of where you were — including arrival and departure days.
- Get advice before you are close to a threshold, not after.
Day-counting conventions differ by jurisdiction: many countries count any day you are physically present for any part of the day, but exceptions for transit and partial days vary. Don't assume one country's convention applies to another.
Counting is the easy part to get right
You can't control how a tax authority weighs your ties — but you can control the one input they will always ask for: how many days, and exactly when. Countly keeps that record automatically, per country, so the number you bring to your adviser is the real one.
This is general information, not tax advice. Residency rules and treaties are specific and change; confirm your situation with a qualified professional.